In motor vehicle passenger compartment heaters of the above type, the current state of the art is to have an equal or nearly equal number of tubes in each group or pass. By doing this, the coolant velocity in all the tubes is nearly identical for what was believed to be best heat transfer performance. However, there can be debris in the coolant system and if this debris lodges in the tube inlets, the entrance velocity into these tubes increases making the heater core more susceptible to erosion.